1. Field of the Invention
The present invention relates to a printing cartridge, and, more particularly, to a printing cartridge having a filter tower assembly and a process for forming the same.
2. Description of the Related Art
A printing cartridge includes a body forming an ink reservoir. One form of a printing cartridge, know as an ink jet printhead cartridge, combines ink storage and drop ejection functions into a unitary package. The ink jet printhead cartridge body has a base for attachment of a printhead. The ink reservoir may include one or more chambers containing an ink-saturated porous material, such as for example, a polyurethane foam. The printhead includes a nozzle plate including a plurality of ink jetting nozzles, fluidic passages and chambers for receiving and transporting ink to the ink jetting nozzles, and selectable electrical components which when actuated cause ink to be ejected from one or more of the ink jetting nozzles.
An interconnection between the ink reservoir and the printhead is provided, at least in part, by a tower, sometimes also referred to as a standpipe, that extends upwardly from the base. In order to prevent the introduction of particulate matter and/or air bubbles into the flow path of the interconnection from the ink reservoir to the ink jetting nozzles of the printhead, a filter is typically attached to the tower, and hence, the tower/filter combination is sometimes also referred to as a filter tower. The filter may be in the form of a fine mesh stainless steel filter affixed to the entrance of the tower. The filter also acts as a capillary drain, allowing ink passage upon demand but preventing air passage into the tower. One known filter attach method uses an adhesive to attach the filter to the tower.
It is known to form the body of an ink jet printhead cartridge from an amorphous polymer. Polymers which are amorphous typically allow for easier joining to other substances, such as a metal. The reason for this is that the amorphous polymers tend to soften when heated to their heat deflection temperatures rather than melting. In contrast, a crystalline or semi-crystalline polymer will tend to melt at a given temperature. One significant difference between the behaviors amorphous polymers and crystalline polymers, for example, is the viscosity of the heated polymer. A softened amorphous polymer still has a very high viscosity, and therefore, the material itself retains a significant amount of strength which aids in joining materials. In contrast, a highly crystalline polymer above its melt temperature drops dramatically in viscosity. Due to this drop in viscosity, the crystalline polymer material does not retain as much strength as a softened amorphous polymer, and therefore, joining a crystalline polymer with another material, such as for example, metal, becomes more complicated.
For example, for an ink jet printhead cartridge made from an amorphous polymer, the stainless steel filter can simply be heated by direct contact with another heated material, such as a copper heating block, and then pressed into the amorphous polymer. The amorphous polymer will soften and under pressure can be extruded through the mesh in the stainless steel filter. While the system is still at the softening temperature of the amorphous polymer the heated block can be retracted, leaving the filter attached to the amorphous polymer. The amorphous polymer retains enough strength to hold the filter mesh in place even while above its softening temperature.
The heat staking process noted above for use with an amorphous printing cartridge body will not provide acceptable results for printing cartridges having a body formed from a crystalline polymer or a semi-crystalline polymer. For example, when the filter is heated and pressed into a crystalline polymer, if the temperature is below the melt temperature, then the crystalline polymer will not melt, nor will it soften enough to extrude through the filter mesh. Upon reaching the polymer melt temperature, the crystalline polymer will indeed melt and flow through the filter mesh; however, it does not have enough strength to hold the filter in place when the heated block is removed. As the melted crystalline polymer flows through the filter mesh and contacts the heated block it will tend to pull up with the heated block when the heated block is retracted, and pull the filter with it. This causes a compromise in the welded interface of the filter to the crystalline polymer. Accordingly, the existing heat staking process of filter attachment is not ideal for printing cartridge bodies formed from crystalline or semi-crystalline polymers.
What is needed in the art is a printing cartridge including a filter tower assembly having a tower formed from a crystalline or semi-crystalline polymer, wherein the filter tower assembly can be formed by a relatively simple, cost-effective and reliable process for attaching the filter, such as a metal mesh filter, to the crystalline or semi-crystalline polymer tower.